1. Field of the Invention
The present invention relates to an electroluminescent (EL) device including a phosphor layer having nitride semiconductor particles.
2. Description of the Related Art
A GaN-based nitride semiconductor has excellent characteristics as light emitting materials and an LED and laser including a single crystal thin layer, have become commercially practical. However, it is difficult to apply the single crystal thin layer to a light emitting device with a large area. On the other hand, as for the nitride semiconductor, various methods of preparing particles are proposed and applications to a light emitting device are expected. As a light emitting device using the nitride semiconductor particle, application to a dispersion type electroluminescent device is expected.
FIG. 4 is a schematic sectional view showing a constitution of a typical dispersion type direct current electroluminescent device 50. This dispersion type direct current electroluminescent device 50 has a transparent electrode 52, a phosphor layer 53 and a rear surface electrode 54, which are stacked in turn on a substrate 51. In the phosphor layer 53, luminescent particles 56 are dispersed in an organic binder 59. The luminescent particles 56 includes, for example, a ZnS:Mn luminescent coated with CuxS. The transparent electrode 52 and the rear surface electrode 54 are electrically connected to a direct current power source 55. When a voltage is applied from the power source 55 between the transparent electrode 52 and the rear surface electrode 54, the luminescent particles 56 in the phosphor layer 53 are caused to emit light. The light passes through the transparent electrode 52 and the substrate 51, and the light is guided from the light emitting device 50, as disclosed in A. G. Fischer, J. Electrochem. Soc., 109, 1043, 1962.
It is known that the nitride semiconductor emits light by recombination of an electron and a hole. In order to improve luminous efficiency, it is necessary to inject these charges such as electrons and holes into the semiconductor particles with efficiency. Therefore, when the nitride semiconductor particle is applied to a dispersion type electroluminescent structure, it is desirable that the nitride semiconductor particles are filled leaving no space between particles as far as possible and charges from the electrode are efficiently transferred and charges are efficiently injected into the particle.
However, in the nitride semiconductor particle, there is a problem that many non-radiative recombination centers are present in the grain boundaries between the particles and charges come to these centers are trapped in the non-radiative recombination centers to interfere with the charge injection into the particle. Therefore, a means for improving the efficiency of charge injection into the particle becomes essential.